Vertebrate skeletal and insect flight myofibrillar structure is being explored by coordinated use of X-ray diffraction, microscopy, biochemistry, and microscopic mass determination methods, with emphasis on the arrangement, number and behavior of crossbridges. We are exploiting and developing better methods of chemical skinning and chemical fixation of muscles, aided by monitoring the low-angle X-ray pattern in rest and rigor. Optical diffraction and processing of EM images is verifying improved fidelity of structural preservation and assisting in structural analysis. Methods successful with frog and waterbug muscle are to be adapted to mammalian and other muscles. 200 Angstrom units thin sections and goniometer electron microscopy are emphasized in analysis of crossbridges, Z band organization. The degree of engagement between myosin heads and actin subunits in rigor will be quantified by photometric mass determination techniques with interference microscopy and electron microscopy. ATP analogues, protein modification and tension variations are being used to expose features of mechanochemical coupling in the crossbridge cycle. Elasticity and lattice dimorphism in the vertebrate Z band are being studied to illuminate its role in mechanical behavior of the sarcomere. Computer modelling of the rigor crossbridge lattice, insect and vertebrate, is underway in collaboration with Haselgrove. Synchrotron X-ray diffraction, electron cryo-microscopy and STEM investigation of insect flight muscle were added to this program during the Principal Investigator's sabbatical at EMBL and the Max Planck Institute in Heidelberg, Germany.